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`Europäisches Patentamt
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`European Patent Office
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`Office européen des brevets
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`*EP001278395A2*
`EP 1 278 395 A2
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`(11)
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`EUROPEAN PATENT APPLICATION
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`(43) Date of publication:
`22.01.2003 Bulletin 2003/04
`
`(21) Application number: 02254939.8
`
`(22) Date of filing: 12.07.2002
`
`(51) Int Cl.7: H04R 3/00
`
`(84) Designated Contracting States:
`AT BE BG CH CY CZ DE DK EE ES FI FR GB GR
`IE IT LI LU MC NL PT SE SK TR
`Designated Extension States:
`AL LT LV MK RO SI
`
`(72) Inventors:
`• Elko, Gary W.
`Summit, New Jersey 07901 (US)
`• Teutsch, Heinz
`90469 Nurnberg (DE)
`
`(30) Priority: 18.07.2001 US 306271 P
`30.10.2001 US 999298
`
`(71) Applicant: Agere Systems Inc.
`Allentown, PA 18109 (US)
`
`(74) Representative: Williams, David John et al
`Page White & Farrer,
`54 Doughty Street
`London WC1N 2LS (GB)
`
`(54)
`
`Second-order adaptive differential microphone array
`
`(57)
`A second-order adaptive differential micro-
`phone array (ADMA) has two first-order elements (e.g.,
`802 and 804 of Fig. 8), each configured to convert a re-
`ceived audio signal into an electrical signal. The ADMA
`also has (i) two delay nodes (e.g., 806 and 808) config-
`ured to delay the electrical signals from the first-order
`elements and (ii) two subtraction nodes (e.g., 810 and
`812) configured to generate forward-facing and back-
`ward-facing cardioid signals based on differences be-
`tween the electrical signals and the delayed electrical
`signals. The ADMA also has (i) an amplifier (e.g., 814)
`configured to amplify the backward-facing cardioid sig-
`nal by a gain parameter; (ii) a third subtraction node (e.
`
`g., 816) configured to generate a difference signal
`based on a difference between the forward-facing car-
`dioid signal and the amplified backward-facing cardioid
`signal; and (iii) a lowpass filter (e.g., 818) configured to
`filter the difference signal from the third subtraction node
`to generate the output signal for the second-order AD-
`MA. The gain parameter for the amplifier can be adap-
`tively adjusted to move a null in the back half plane of
`the ADMA to track a moving noise source. In a subband
`implementation, a different gain parameter can be adap-
`tively adjusted to move a different null in the back half
`plane to track a different moving noise source for each
`different frequency subband.
`
`Printed by Jouve, 75001 PARIS (FR)
`
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`EP1 278 395A2
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`Amazon v. Jawbone
`U.S. Patent 8,280,072
`Amazon Ex. 1004
`
`

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`EP 1 278 395 A2
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`Description
`
`BACKGROUND OF THE INVENTION
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`Field of the Invention
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`[0001] The present invention relates to microphone arrays that employ directionality characteristics to differentiate
`between sources of noise and desired sound sources.
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`Description of the Related Art
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`[0002] The presence of background noise accompanying all kinds of acoustic signal transmission is a ubiquitous
`problem. Speech signals especially suffer from incident background noise, which can make conversations in adverse
`acoustic environments virtually impossible without applying appropriately designed electroacoustic transducers and
`sophisticated signal processing. The utilization of conventional directional microphones with fixed directivity is a limited
`solution to this problem, because the undesired noise is often not fixed to a certain angle.
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`SUMMARY OF THE INVENTION
`
`[0003] Embodiments of the present invention are directed to adaptive differential microphone arrays (ADMAs) that
`are able to adaptively track and attenuate possibly moving noise sources that are located in the back half plane of the
`array. This noise attenuation is achieved by adaptively placing a null into the noise source's direction of arrival. Such
`embodiments take advantage of the adaptive noise cancellation capabilities of differential microphone arrays in com-
`bination with digital signal processing. Whenever undesired noise sources are spatially non-stationary, conventional
`directional microphone technology has its limits in terms of interference suppression. Adaptive differential microphone
`arrays (ADMAs) with their null-steering capabilities promise better performance.
`[0004]
`In one embodiment, the present invention is a second-order adaptive differential microphone array (ADMA),
`comprising (a) a first first-order element (e.g., 802 of Fig. 8) configured to convert a received audio signal into a first
`electrical signal; (b) a second first-order element (e.g., 804 of Fig. 8) configured to convert the received audio signal
`into a second electrical signal; (c) a first delay node (e.g., 806 of Fig. 8) configured to delay the first electrical signal
`from the first first-order element to generate a delayed first electrical signal; (d) a second delay node (e.g., 808 of Fig.
`8) configured to delay the second electrical signal from the second first-order element to generate a delayed second
`electrical signal; (e) a first subtraction node (e.g., 810 of Fig. 8) configured to generate a forward-facing cardioid signal
`based on a difference between the first electrical signal and the delayed second electrical signal; (f) a second subtraction
`node (e.g., 812 of Fig. 8) configured to generate a backward-facing cardioid signal based on a difference between the
`second electrical signal and the delayed first electrical signal; (g) an amplifier (e.g., 814 of Fig. 8) configured to amplify
`the backward-facing cardioid signal by a gain parameter to generate an amplified backward-facing cardioid signal; and
`(h) a third subtraction node (e.g., 816 of Fig. 8) configured to generate a difference signal based on a difference between
`the forward-facing cardioid signal and the amplified backward-facing cardioid signal.
`[0005]
`In another embodiment, the present invention is an apparatus for processing signals generated by a micro-
`phone array (ADMA) having (i) a first first-order element (e.g., 802 of Fig. 8) configured to convert a received audio
`signal into a first electrical signal and (ii) a second first-order element (e.g., 804 of Fig. 8) configured to convert the
`received audio signal into a second electrical signal, the apparatus comprising (a) a first delay node (e.g., 806 of Fig.
`8) configured to delay the first electrical signal from the first first-order element to generate a delayed first electrical
`signal; (b) a second delay node (e.g., 808 of Fig. 8) configured to delay the second electrical signal from the second
`first-order element to generate a delayed second electrical signal; (c) a first subtraction node (e.g., 810 of Fig. 8)
`configured to generate a forward-facing cardioid signal based on a difference between the first electrical signal and
`the delayed second electrical signal; (d) a second subtraction node (e.g., 812 of Fig. 8) configured to generate a
`backward-facing cardioid signal based on a difference between the second electrical signal and the delayed first elec-
`trical signal; (e) an amplifier (e.g., 814 of Fig. 8) configured to amplify the backward-facing cardioid signal by a gain
`parameter to generate an amplified backward-facing cardioid signal; and (g) a third subtraction node (e.g., 816 of Fig.
`8) configured to generate a difference signal based on a difference between the forward-facing cardioid signal and the
`amplified backward-facing cardioid signal.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`[0006] Other aspects, features, and advantages of the present invention will become more fully apparent from the
`following detailed description, the appended claims, and the accompanying drawings in which:
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`EP 1 278 395 A2
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`Fig. 1 shows a schematic representation of a first-order adaptive differential microphone array (ADMA) receiving
`an audio signal from a signal source at a distance where farfield conditions are applicable;
`Fig. 2 shows a schematic diagram of a first-order fullband ADMA based on an adaptive back-to-back cardioid
`system;
`Fig. 3 shows the directivity pattern of the first-order ADMA of Fig. 2;
`Fig. 4 shows directivity patterns that can be obtained by the first-order ADMA for θ1 values of 90°, 120°, 150°, and
`180°;
`Fig. 5 shows a schematic diagram of a second-order fullband ADMA;
`Fig. 6 shows the directivity pattern of a second-order back-to-back cardioid system;
`Fig. 7 shows the directivity patterns that can be obtained by a second-order ADMA formed from two dipole elements
`for θ22 values of 90°, 120°, 150°, and 180°;
`Fig. 8 shows a schematic diagram of a subband two-element ADMA;
`Figs. 9A and 9B depict the fullband ADMA directivity patterns for first-order and second-order arrays, respectively;
`and
`Figs. 10 and 11 show measured directivity of first- and second-order subband implementations of the ADMA of
`Fig. 8, respectively, for four simultaneously playing sinusoids.
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`DETAILED DESCRIPTION
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`First-Order Fullband ADMA
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`[0007] Fig. 1 shows a schematic representation of a first-order adaptive differential microphone array (ADMA) 100
`receiving audio signal s(t) from audio source 102 at a distance where farfield conditions are applicable. When farfield
`conditions apply, the audio signal arriving at ADMA 100 can be treated as a plane wave. ADMA 100 comprises two
`zeroth-order microphones 104 and 106 separated by a distance d. Electrical signals generated by microphone 106
`are delayed by inter-element delay T at delay node 108 before being subtracted from the electrical signals generated
`by microphone 104 at subtraction node 110 to generate the ADMA output y(t). The magnitude of the frequency and
`angular dependent response H1(f,θ) of first-order ADMA 100 for a signal point source at a distance where farfield
`conditions are applicable can be written according to Equation (1) as follows:
`
`where Y1(ƒ,θ) is the spectrum of the ADMA output signal y(t), S(ƒ) is the spectrum of the signal source, k is the sound
`vector, |k| = k = 2πƒ / cis the wavenumber, c is the speed of sound, and d is the displacement vector between micro-
`phones 104 and 106. As indicated by the term Y1(ƒ,θ), the ADMA output signal is dependent on the angle θ between
`the displacement vector d and the sound vector k as well as on the frequency ƒ of the audio signal s(t).
`[0008] For small element spacing and short inter-element delay (kd «π and T «1 / 2ƒ), Equation (1) can be approx-
`imated according to Equation (2) as follows:
`
`As can be seen, the right side of Equation (2) consists of a monopole term and a dipole term (cosθ). Note that the
`amplitude response of the first-order differential array rises linearly with frequency. This frequency dependence can
`be corrected for by applying a first-order lowpass filter at the array output. The directivity response can then be ex-
`pressed by Equation (3) as follows:
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`EP 1 278 395 A2
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`Since the location of the source 102 is not typically known, an implementation of a first-order ADMA based on Equation
`(3) would need to involve the ability to generate any time delay T between the two microphones. As such, this approach
`is not suitable for a real-time system. One way to avoid having to generate the delay T directly in order to obtain the
`desired directivity response is to utilize an adaptive back-to-back cardioid system
`[0009] Fig. 2 shows a schematic diagram of a first-order fullband ADMA 200 based on an adaptive back-to-back
`cardioid system. In ADMA 200, signals from both microphones 202 and 204 are delayed by a time delay T at delay
`nodes 206 and 208, respectively. The delayed signal from microphone 204 is subtracted from the undelayed signal
`from microphone 202 at forward subtraction node 210 to form the forward-facing cardioid signal cF(t). Similarly, the
`delayed signal from microphone 202 is subtracted from the undelayed signal from microphone 204 at backward sub-
`traction node 212 to form the backward-facing cardioid signal cB(t), which is amplified by gain β at amplifier 214. The
`signal y(t) is generated at subtraction node 216 based on the difference between the forward and amplified backward
`signals. The signal y(t) is then lowpass filtered at filter 218 to generate the ADMA output signal yout(t).
`[0010] Fig. 3 shows the directivity pattern of the first-order back-to-back cardioid system of ADMA 200. ADMA 200
`can be used to adaptively adjust the response of the backward facing cardioid in order to track a possibly moving noise
`source located in the back half plane. By choosing T = d / c , the back-to-back cardioid can be formed directly by
`appropriately subtracting the delayed microphone signals.
`[0011] The transfer function H1(ƒ,θ) of first-order ADMA 200 can be written according to Equation (4) as follows:
`
`where Yout(ƒ,θ) is the spectrum of the ADMA output signal yout(t).
`[0012] The single independent null angle θ1 of first-order ADMA 200, which, for the present discussion, is assumed
`to be placed into the back half plane of the array (90° ≤ θ1 ≤ 180°), can be found by setting Equation (4) to zero and
`solving for θ = θ1, which yields Equation (5) as follows:
`
`which for small spacing and short delay can be approximated according to Equation (6) as follows:
`
`θ1 ≈ arccos β - 1
`-------------,
`β + 1
`
`(6)
`
`where 0 ≤ β ≤ 1 under the constraint (90° ≤ θ1 ≤ 180°). Fig. 4 shows the directivity patterns that can be obtained by
`first-order ADMA 200 for θ1 values of 90°, 120°, 150°, and 180°.
`[0013]
`In a time-varying environment, an adaptive algorithm is preferably used in order to update the gain parameter
`β. In one implementation, a normalized least-mean-square (NLMS) adaptive algorithm may be utilized, which is com-
`putationally inexpensive, easy to implement, and offers reasonably fast tracking capabilities. One possible real-valued
`time-domain one-tap NLMS algorithm can be written according to Equation2 (7a) and (7b) as follows:
`
`y(i) = cF(i)-β(i)cB(i),
`
`(7a)
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`where cF(i) and cB(i) are the values for the forward- and backward-facing cardioid signals at time instance i, µ is an
`adaptation constant where 0 < µ < 2, and a is a small constant where a > 0.
`[0014] Further information on first-order adaptive differential microphone arrays is provided in U.S. Patent No.
`5,473,701 (Cezanne et al.), the teachings of which are incorporated herein by reference.
`
`Second-Order Fullband ADMA
`
`[0015] Fig. 5 shows a schematic diagram of a second-order fullband ADMA 500 comprising two first-order ADMAs
`502 and 504, each of which is an instance of first-order ADMA 100 of Fig. 1 having an inter-element delay T1. After
`delaying the signal from first-order array 504 by an additional time delay T2 at delay node 506, the difference between
`the two first=order signals is generated at subtraction node 508 to generate the output signal y2(t) of ADMA 500.
`[0016] When farfield conditions apply, the magnitude of the frequency and angular dependent response H2(ƒ,θ) of
`second-order ADMA 500 is given by Equation (8) as follows:
`
`where Y2(ƒ, θ) is the spectrum of the ADMA output signal y2(t). For the special case of small spacing and delay, i.e.,
`kd1, kd2 « π and T1, T2 «1/2ƒ, Equation (8) may be written as Equation (9) as follows:
`
`Analogous to the case of first-order differential array 200 of Fig. 2, the amplitude response of second-order array 500
`consists of a monopole term, a dipole term (cosθ), and an additional quadrapole term (cos2θ). Also, a quadratic rise
`as a function of frequency can be observed. This frequency dependence can be equalized by applying a second-order
`lowpass filter. The directivity response can then be expressed by Equation (10) as follows:
`
`which is a direct result of the pattern multiplication theorem in electroacoustics.
`[0017] One design goal of a second-order differential farfield array, such as ADMA 500 of Fig. 5, may be to use the
`array in a host-based environment without the need for any special purpose hardware, e.g., additional external DSP
`interface boards. Therefore, two dipole elements may be utilized in order to form the second-order array instead of
`⬅0 which means that one null angle is fixed to θ21 = 90°. In this
`four omnidirectional elements. As a consequence, T1
`case, although two independent nulls can be formed by the second-order differential array, only one can be made
`adaptive if two dipole elements are used instead of four omnidirectional transducers. The implementation of such a
`second-order ADMA may be based on first-order cardioid ADMA 200 of Fig. 2, where d = d2, T = T2,β= β2, and d1 is
`the acoustical dipole length of the dipole transducer. Additionally, the lowpass filter is chosen to be a second-order
`lowpass filter. Fig. 6 shows the directivity pattern of such a second-order back-to-back cardioid system. Those skilled
`in the art will understand that a second-order ADMA can also be implemented with three omnidirectional elements.
`[0018] The transfer function H2(ƒ,θ) of a second-order ADMA formed of two dipole elements can be written according
`to Equation (11) as follows:
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`EP 1 278 395 A2
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`with null angles given by Equations (12a) and (12b) as follows:
`
`θ21 = 90°,
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`θ22 ≈ arccos
`
`β2 - 1
`----------------,
`β2 + 1
`
`(12a)
`
`(13b)
`
`where 0 ≤ β2 ≤1 under the constraint 90 ° ≤ β22 ≤ 180°. Fig. 7 shows the directivity patterns that can be obtained by a
`second-order ADMA formed from two dipole elements for θ22 values of 90°, 120°, 150°, and 180°.
`[0019] As shown in Elko, G. W., "Superdirectional Microphone Arrays," Acoustic Signal Processing for Telecommu-
`nication, J. Benesty and S. L. Gay (eds.), pp. 181-236, Kluwer Academic Publishers, 2000, a second-order differential
`array is typically superior to a first-order differential array in terms of directivity index, front-to-back ratio, and beamwidth.
`
`Subband ADMA
`
`[0020] Fig. 8 shows a schematic diagram of a subband two-element ADMA 800 comprising two elements 802 and
`804. When elements 802 and 804 are omnidirectional elements, ADMA 800 is a first-order system; when elements
`802 and 804 are dipole elements, ADMA 800 is a second-order system. ADMA 800 is analogous to fullband ADMA
`200 of Fig. 2, except that one additional degree of freedom is obtained for ADMA 800 by performing the adaptive
`algorithm independently in different frequency subbands. In particular, delay nodes 806 and 808 of subband ADMA
`800 are analogous to delay nodes 206 and 208 of fullband ADMA 200; subtraction nodes 810, 812, and 816 of ADMA
`800 are analogous to subtraction nodes 210, 212, and 216 of ADMA 200; amplifier 814 of ADMA 800 is analogous to
`amplifier 214 of ADMA 200; and lowpass filter 818 of ADMA 800 is analogous to lowpass filter 218 of ADMA 200,
`except that, for ADMA 800, the processing is independent for different frequency subbands.
`[0021] To provide subband processing, analysis filter banks 820 and 822 divide the electrical signals from elements
`802 and 804, respectively, into two or more subbands l, and amplifier 814 can apply a different gain β(l,i) to each
`different subband l in the backward-facing cardioid signal cB(l,i). In addition, synthesis filter bank 824 combines the
`different subband signals y(l,i) generated at summation node 816 into a single fullband signal y(t), which is then lowpass
`filtered by filter 818 to generate the output signal yout(t) of ADMA 800.
`[0022] The gain parameter β(l,i), where l denotes the subband bin and i is the discrete time instance, is preferably
`updated by an adaptive algorithm that minimizes the output power of the array. This update therefore effectively adjusts
`the response of the backward-facing cardioid cB(l,i) and can be written according to Equations (13a) and (13b) as
`follows;
`
`y(l,i)= cF(l,i) - β(l,i)cB(l,i),
`
`(13a)
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`and µ is the update parameter and a is a positive constant.
`[0023] By using this algorithm, multiple spatially distinct noise sources with non-overlapping spectra located in the
`back half plane of the ADMA can be tracked and attenuated simultaneously.
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`Implementation and Measurements
`
`[0024] PC-based real-time implementations running under the Microsoft™ Windows™ operating system were real-
`ized using a standard soundcard as the analog-to-digital converter. For these implementations, the demonstrator's
`analog front-end comprised two omnidirectional elements of the type Panasonic WM-54B as well as two dipole elements
`of the type Panasonic WM-55D103 and a microphone preamplifier offering 40-dB gain comprise the analog front-end.
`The implementations of the first-order ADMAs of Figs. 2 and 8 utilized the two omnidirectional elements and the pream-
`plifier, while the implementation of the second-order ADMA of Fig. 5 utilized the two dipole elements and the pream-
`plifier.
`[0025] The signals for the forward-facing cardioids CF (t) and the backward-facing cardioids cB (t) of the first-order
`ADMAs of Figs. 2 and 8 were obtained by choosing the spacing d between the omnidirectional microphones such that
`there is one sample delay between the corresponding delayed and undelayed microphone signals. Similarly, the signals
`for the forward- and backward-facing cardioids of the second-order ADMA of Fig. 5 were obtained by choosing the
`spacing d2 between the dipole microphones such that there is one sample delay between the corresponding delayed
`and undelayed microphone signals. Thus, for example, for a sampling frequency ƒs of 22050 Hz, the microphone
`spacing d = d2 =1.54 cm. For the Panasonic dipole elements, the acoustical dipole length d1 was found to be 0.8 cm.
`[0026] Figs. 9A and 9B depict the fullband ADMA directivity patterns for first-order and second-order arrays, respec-
`tively. These measurements were performed by placing a broadband jammer (noise source) at approximately 90° with
`respect to the array's axis (i.e., θ1 for the first-order array and θ22 for the second-order array) utilizing a standard
`directivity measurement technique. It can be seen that deep nulls covering wide frequency ranges are formed in the
`direction of the jammer.
`[0027] Figs. 10 and 11 show measured directivity of first- and second-order subband implementations of ADMA 800
`of Fig. 8, respectively, for four simultaneously playing sinusoids. For the first-order subband implementation, four loud-
`speakers simultaneously played sinusoidal signals while positioned in the back half plane of the arrays at θ1 values of
`approximately 90°, 120°, 150°, and 180°. For the second-order subband implementation, four loudspeakers simulta-
`neously played sinusoidal signals while positioned in the back half plane of the arrays at θ22 values of approximately
`110°, 120°, 150°, and 180°. As can be seen, these measurements are in close agreement with the simulated patterns
`shown in Figs. 4 and 7.
`[0028]
`In order to combat the noise amplification properties inherent in differential arrays, the demonstrator included
`a noise reduction method as presented in Diethorn, E. J., "A Subband Noise-Reduction Method for Enhancing Speech
`in Telephony & Teleconferencing," IEEE Workshop on Applications of Signal Processing to Audio and Acoustics, Mo-
`honk, USA, 1997, the teachings of which are incorporated herein by reference.
`
`Conclusions
`
`[0029] First- and second-order ADMAs which are able to adaptively track and attenuate a possibly moving noise
`source located in the back half plane of the arrays have been presented. It has been shown that, by performing the
`calculations in subbands, even multiple spatially distinct noise sources with non-overlapping spectra can be tracked
`and attenuated simultaneously. The real-time implementation presents the dynamic performance of the ADMAs in real
`acoustic environments and shows the practicability of using these arrays as acoustic front-ends for a variety of appli-
`cations including telephony, automatic speech recognition, and teleconferencing.
`[0030] The present invention may be implemented as circuit-based processes, including possible implementation
`on a single integrated circuit. As would be apparent to one skilled in the art, various functions of circuit elements may
`also be implemented as processing steps in a software program. Such software may be employed in, for example, a
`digital signal processor, micro-controller, or general-purpose computer.
`[0031] The present invention can be embodied in the form of methods and apparatuses for practicing those methods.
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`The present invention can also be embodied in the form of program code embodied in tangible media, such as floppy
`diskettes, CD-ROMs, hard drives, or any other machine-readable storage medium, wherein, when the program code
`is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the
`invention. The present invention can also be embodied in the form of program code, for example, whether stored in a
`storage medium, loaded into and/or executed by a machine, or transmitted over some transmission medium or carrier,
`such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the pro-
`gram code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for
`practicing the invention. When implemented on a general-purpose processor, the program code segments combine
`with the processor to provide a unique device that operates analogously to specific logic circuits.
`[0032] The use of figure reference labels in the claims is intended to identify one or more possible embodiments of
`the claimed subject matter in order to facilitate the interpretation of the claims. Such labeling is not to be construed as
`necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures.
`[0033]
`It will be further understood that various changes in the details, materials, and arrangements of the parts
`which have been described and illustrated in order to explain the nature of this invention may be made by those skilled
`in the art without departing from the scope of the invention as expressed in the following claims.
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`1. A second-order adaptive differential microphone array (ADMA), comprising:
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`(a) a first first-order element (e.g., 802 of Fig. 8) configured to convert a received audio signal into a first
`electrical signal;
`(b) a second first-order element (e.g., 804 of Fig. 8) configured to convert the received audio signal into a
`second electrical signal;
`(c) a first delay node (e.g., 806 of Fig. 8) configured to delay the first electrical signal from the first first-order
`element to generate a delayed first electrical signal;
`(d) a second delay node (e.g., 808 of Fig. 8) configured to delay the second electrical signal from the second
`first-order element to generate a delayed second electrical signal;
`(e) a first subtraction node (e.g., 810 of Fig. 8) configured to generate a forward-facing cardioid signal based
`on a difference between the first electrical signal and the delayed second electrical signal;
`(f) a second subtraction node (e.g., 812 of Fig. 8) configured to generate a backward-facing cardioid signal
`based on a difference between the second electrical signal and the delayed first electrical signal;
`(g) an amplifier (e.g., 814 of Fig. 8) configured to amplify the backward-facing cardioid signal by a gain pa-
`rameter to generate an amplified backward-facing cardioid signal; and
`(h) a third subtraction node (e.g., 816 of Fig. 8) configured to generate a difference signal based on a difference
`between the forward-facing cardioid signal and the amplified backward-facing cardioid signal.
`
`2. An apparatus for processing signals generated by a microphone array (ADMA) having (i) a first first-order element
`(e.g., 802 of Fig. 8) configured to convert a received audio signal into a first electrical signal and (ii) a second first-
`order element (e.g., 804 of Fig. 8) configured to convert the received audio signal into a second electrical signal,
`the apparatus comprising:
`
`(a) a first delay node (e.g., 806 of Fig. 8) configured to delay the first electrical signal from the first first-order
`element to generate a delayed first electrical signal;
`(b) a second delay node (e.g., 808 of Fig. 8) configured to delay the second electrical signal from the second
`first-order element to generate a delayed second electrical signal;
`(c) a first subtraction node (e.g., 810 of Fig. 8) configured to generate a forward-facing cardioid signal based
`on a difference between the first electrical signal and the delayed second electrical signal;
`(d) a second subtraction node (e.g., 812 of Fig. 8) configured to generate a backward-facing cardioid signal
`based on a difference between the second electrical signal and the delayed first electrical signal;
`(e) an amplifier (e.g., 814 of Fig. 8) configured to amplify the backward-facing cardioid signal by a gain pa-
`rameter to generate an amplified backward-facing cardioid signal; and
`(g) a third subtraction node (e.g., 816 of Fig. 8) configured to generate a difference signal based on a difference
`between the forward-facing cardioid signal and the amplified backward-facing cardioid signal.
`
`3. The invention of either claims 1 or 2, wherein each of the first and second first-order elements is a first-order
`differential microphone array (e.g., 100 of Fig. 1).
`
`8
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`

`

`4. The invention of claim 3, wherein each first-order differential microphone array comprises:
`
`EP 1 278 395 A2
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`(1) a first omnidirectional element (e.g., 104 of Fig. 1) configured to convert the received audio signal into an
`electrical signal;
`(2) a second omnidirectional element (e.g., 106 of Fig. 1) configured to convert the received audio signal into
`an electrical signal;
`(3) a delay node (e.g., 108 of Fig. 1) configured to delay the electrical signal from the second omnidirectional
`element to generate a delayed electrical signal; and
`(4) a first subtraction node (e.g., 110 of Fig. 1) configured to generate the corresponding electrical signal for
`the first-order element based on a difference between the electrical signal from the first omnidirectional element
`and the delayed electrical signal from the delay node.
`
`5. The invention of either claims 1 or 2, wherein the gain parameter for the amplifier is configured to be adaptively
`adjusted to move a null located in a back half plane of the second-order ADMA to track a moving noise source.
`
`6. The invention of claim 5, wherein the gain parameter is configured to be adaptively adjusted to minimize output
`power from the second-order ADMA.
`
`7. The invention of either claims 1 or 2, further comprising:
`
`(i) a first analysis filter bank (e.g., 820 of Fig. 8) configured to divide the first electrical signal from the first first-
`order element into two or more subband electrical signals corresponding to two or more different frequency
`subbands;
`(j) a second analysis filter bank (e.g., 822 of Fig. 8) configured to divide the second electrical signal from the
`second first-order element into two or more subband electrical signals corresponding to the two or more dif-
`ferent frequency subbands; and
`(k) a synthesis filter bank (e.g., 824 of Fig. 8) configured to combine two or more different subband difference
`signals generated by the third difference node to form a fullband difference signal.
`
`8. The invention of claim 7, wherein the amplifier is configured to apply a different subband gain parameter to a
`backward-facing subband cardioid signal generated by the second subtraction node for each different frequency
`subband.
`
`9. The invention of claim 8, wherein each different subband gain parameter is configured to be adaptively adjusted
`to move a different null in a back half plane of the second-order ADMA to track a different moving noise source
`corresponding to each different frequency subband.
`
`10. The invention of claim 9, wherein each different subband gain parameter is configured to be adaptively adjusted
`to minimize output power from the second-order ADMA in the corresponding frequency subband.
`
`5
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`10
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`20
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`30
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`40
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`EP 1 278 395 A2
`EP 1 278 395 A2
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`FIG. 1
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`EP 1 278 395 A2
`EP 1 278 395 A2
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`FIG. 2
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`EP 1 278 395 A2
`EP 1 278 395 A2
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`FIG.
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`4
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`FIG. 3
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`EP 1 278 395 A2
`EP 1 278 395 A2
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`FIG. 6
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`—— FORWARD FACING CARDIOID
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`~~~ BACKWARD FACING CARDIOID
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`EP 1 278 395 A2
`EP 1 278 395 A2
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`FIG. 7
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